In regard to plastic members that require characteristics of being lightweight and having high strength, and have complex shapes, so-called FRP (Fiber Reinforced Plastics; also referred to as fiber-reinforced composite materials) are conventionally used for the forming of component parts of fishing ships, sports goods, bathtubs, automobiles, and the like.
In recent years, among these FRP materials, sheet-like or bulk-like materials that use short fibers as reinforcing materials, namely, so-called SMCs (sheet-molding compounds) and BMCs (bulk molding compounds), have been increasingly utilized from the viewpoints of workability, working environment, and the like. Regarding the curable resins that constitute SMCs and BMCs, unsaturated polyesters and materials obtained by diluting oligomers such as vinyl esters with styrene are generally used, and if necessary, a curing agent and a thickening agent are selected and incorporated thereinto. Furthermore, according to the use applications, a colorant, a low constrictive agent, a mold releasing agent, a filler, and the like can also be added to the curable resin.
Patent Literatures 1 to 4 describe SMCs and BMCs that use epoxy resins.
Meanwhile, fiber-reinforced composite materials (FRP) formed from reinforcing fibers and matrix resins have been widely used for aircraft, automobile, and industrial applications, due to their excellent mechanical properties and the like. In recent years, as the usage results accumulate, the range of applications of fiber-reinforced composite materials is becoming even broader. The matrix resin that constitutes such a composite material is required to have excellent moldability and to exhibit superior mechanical strength even in a high-temperature environment. Regarding the matrix resin, thermosetting resins having excellent impregnating properties or heat resistance are used in many occasions, and a phenolic resin, a melamine resin, a bismaleimide resin, an unsaturated polyester resin, an epoxy resin, or the like is used as such a thermosetting resin. Among these, an epoxy resin has excellent thermal resistance and moldability, and when an epoxy resin is used, a fiber-reinforced composite material having superior mechanical strength is obtained. Therefore, epoxy resins are widely used.
A fiber-reinforced composite material is produced by autoclave molding filament winding molding, resin infusion molding, vacuum resin infusion molding, press molding or the like, using an intermediate material containing reinforcing fibers and a matrix resin composition. Above all, press molding is accompanied by high productivity, and a molded product having excellent design surfaces may be easily obtained. Therefore, the demand for press molding has been increasing in recent years.
Particularly, since a molded product having a complex shape can be easily produced by pres molding a SMC (sheet-molding compound), utilization of fiber-reinforced composite materials in, for example, structural members for automobiles is becoming popular.
Regarding curable resins that constitute SMCs, unsaturated polyesters and materials obtained by diluting oligomers such as vinyl esters with styrene are generally used; however, since these undergo significant cure shrinkage, development of a SMC that uses an epoxy resin as a base resin is desired.
Here, regarding epoxy resin compositions that are used for adhesives, for example, the following have been suggested.
A resin composition including (A) an epoxy resin; (B) an amine-based curing agent; and (C) an accelerator having at least one functional group selected from a dimethylureido group, an imidazole group, and a tertiary amino group, the resin composition being liquid at normal temperature without substantially including a solvent (Patent Literature 5),
a one-liquid heating-curable epoxy resin composition including an epoxy resin; a curing agent including dicyandiamide; a first curing accelerator including 3,4-dichlorophenyl-1,1-dimethylurea; and a second curing accelerator including an imidazole compound having a triazine ring (Patent Literature 6), and
a one-component-based heating-curable epoxy resin composition including (A) an epoxy compound; (B) a curing agent composition obtainable by reacting an amine compound with an epoxy compound; and (C) a filler (Patent Literature 7).